This invention relates to an improved method and apparatus for RF admittance measurement of the degree to which a pipe is filled with fluid.
Pumps of many types will be damaged if they are operated without fluid in them. To protect the expensive pumps, it is desirable to monitor the pump inlet lines and to shut off the pump motor if the inlet line is not filled with fluid.
Monitoring attempts have included RF admittance responding sensors comprising a tubular probe electrode adapted to form a portion of the pump inlet line. The admittance between the probe electrode and the adjacent sections of grounded inlet line varies with the degree to which the inlet line is filled with fluid and with the electrical characteristics of any fluid present.
The fluid being pumped may at one time be an insulator of low dielectric constant, and at another time be extremely conductive for instance when a caustic solution is used to clean the residue of a previously pumped organic fluid.
The fluid being pumped may also be extremely viscous, and leave substantial coatings on the inside of the sensor when the inlet line is not filled. These fluid characteristics have caused the prior art admittance sensing schemes to give unreliable indications of whether a pump inlet line is fluid filled, because there are no known admittance-responsive circuits which can reliably detect a capacitance change on the order of one picofarad due to filling a pipe with an insulator having a low dielectric constant and which can also ignore a resistance on the order of one ohm due to a conductive coating in an otherwise empty sensor.
In U.S. Pats. No. 3,706,980-Maltby and U.S. Pat. No. 3,879,644-Maltby, both assigned to the assignee of this invention, systems are disclosed for mitigating the effects of conductive coatings on admittance-responding probes. This is accomplished by providing a guard electrode which is exposed to the material being measured and driven at substantially the same potential as the probe electrode so as to maintain an accumulated coating at substantially the same potential as the probe electrode and thereby reduces the effect of such a coating on a capacitance measurement. The probes described in these patents are well suited to detecting the level of materials in tanks or bins, but are inadequate for detecting whether the inlet line of a pump is filled. Were the elongated structure of FIG. 1 of these patents utilized to detect fluid in a pipe, either the rod-shaped probe electrode would protrude into and thus interfere with the flow of fluid in the pipe, or if mounted in a recess in the wall of the pipe so as to not protrude into the fluid flow, would be subject to increased coating problems and greatly reduced sensitivity to the presence of material in the pipe itself. Were a plate-like structure as in FIGS. 3 or 5 of U.S. Pat. No. 3,879,644 utilized to detect whether a pipe were filled with fluid, such structures would have to be curved to conform to the pipe radius in order to prevent intrusion into the fluid flow or mounted in a recess in which a substantial amount of material could adhere and foul the probe when the pipe was not filled. Such a curved probe would be very expensive to produce, and in order to reliably detect that a pipe was filled, an array of them would have to be positioned around the pipe, further increasing the cost. Finally, it would be difficult to manufacture the probes depicted in the aforementioned patents in sizes small enough for use with pump inlet pipes, which pipes may be as small as one inch diameter.
Known admittance-responsive circuits for use with guarded probes are also inadequate for use with filled-pipe probes due to the range of impedances presented by the probe, as mentioned above. A conductive coating which would present a resistance on the order of one ohm between the probe electrode and ground of the prior art unguarded tubular probes would present resistances on the same order to the probe electrode and the guard electrodes, and between the guard electrodes and ground. Guard sources which can maintain their output at substantially the same voltage as the probe electrode with a one ohm load do not exist in available admittance-measuring systems. Given such a guard source, known methods of connecting the guard source to the guard electrodes would not permit the guard electrodes to be at the same voltage as the probe due to the inductive impedance of the connection. Even if the guard electrodes themselves could be maintained at probe potential, the portion of a coating over the guard electrodes not in contact with the electrodes will be at a reduced potential due to the coating resistance, and this reduced-potential portion will couple a sufficient fraction of the ohm-range probe electrode to guard electrode coating resistance into the admittance measuring circuit to cause erroneous indications in known admittance measuring circuits. The present invention is directed at surmounting these and other shortcomings of the prior art.